The propulsion of spacecraft in space (where terrestrial gravitation becomes negligible) requires low thrusts (low flow of ejected material), but high ejection speeds of “fuel” to minimize the on-board mass. In fact, the speed increase Δu of a spacecraft is linked to the gas ejection speed ue and to the initial and final masses, m0 and mf, of fuel by the following equation, called the “rocket equation”:
            Δ      ⁢                          ⁢      u        =                            u          e                ⁢                  ln          ⁡                      (                                          m                                  0                  ⁢                                                                                                                    m                f                                      )                              ❘        _
A high gas ejection speed is therefore imperative if it is desired to save fuel. Plasma motors allow these high ejection speeds to be attained. Two quantities are used to characterize a motor, the specific impulse:
            I      s        =                  u        e                    g        o              _expressed in seconds, where go is the gravity constant at the surface of the earth, and the thrust:T={dot over (m)}ue where {dot over (m)} is the mass flow rate.
The principle of plasma motors, described in the illustrated schema of FIG. 1, is the following: the “fuel” (gas) X is first ionized in a plasma to form positive ions X+ and electrons e−, then ejected by being accelerated in an electric field E (often created by accelerating grids), before being neutralized by an additional electron beam Fe− positioned downstream of the acceleration zone. The neutralization is indispensable to prevent the spacecraft becoming electrically charged.
The various prototypes of plasma motors existing to date use, generally speaking, an ionization stage to generate a source of positively charged matter (positive ions), an acceleration stage and a neutralization structure. The ionization sources and the accelerating and neutralizing structures may be varied. But all the motors existing to date use only positively charged matter (positive ions) for propulsion, the negative charge (the electrons) serving solely for the ionization and the neutralization.